In many industrial settings, control systems are used to monitor and control inventories, processes, and the like. Often, such control systems have a centralized control room, with computer systems having user inputs and outputs and having peripheral systems that are known in the art, such as printers, scanners, and the like. The controller and process subsystems are coupled to the computer systems.
Often control systems are distributed such that field devices are separated or geographically removed from the control room. The process subsystem is connected to the field devices. As used herein, the term “field device” encompasses any device that performs a function in a distributed control system and is known in the control art.
Generally, each field device includes a transducer. A transducer is understood to mean either a device that generates an output signal based on a physical input or that generates a physical output based on an input signal. The transducer transforms an input into an output having a different form. Types of transducers include various analytical equipment, pressure sensors, thermistors, thermocouples, strain gauges, flow transmitters, positioners, actuators, solenoids, indicator lights, and the like.
Traditionally, analog field devices have been connected to the process subsystem and the control room by two-wire twisted-pair current loops, with each device connected to the control room by a single two-wire twisted pair loop. Typically, a voltage differential is maintained between the two wires of approximately 20 to 25 volts, and a current between 4 and 20 milliamps (mA) runs through the loop. An analog field device transmits a signal to the control room by modulating the current running through the current loop to a current proportional to the sensed process variable. An analog field device that performs an action under the control of the control room is controlled by the magnitude of the current through the loop, which is modulated by the ports of the process subsystem under the control of the controller.
Traditional discrete devices transmit or respond to a binary signal. Typically, discrete devices operate with a 24-volt signal (AC or DC), a 110 or 240 volt AC signal, or a 5 volt DC signal. Of course, a discrete device may be designed to operate according to any electrical specification required by the control environment.
While historically field devices were capable of performing only one function, more recently hybrid systems that superimpose digital data on the current loop have been used in distributed control systems. The Highway Addressable Remote Transducer (HART) and the Instrument Society of America (ISA) Fieldbus SP50 standards superimpose a digital carrier signal on the current loop signal. The digital carrier signal can be used to send secondary and diagnostic information. Examples of information provided over the carrier signal include secondary process variables, diagnostic information (such as sensor diagnostics, device diagnostics, wiring diagnostics, process diagnostics, and the like), operating temperatures, sensor temperature, calibration data, device ID numbers, configuration information, and so on. Accordingly, a single field device may have a variety of input and output variables and may implement a variety of functions.
Typically, remote applications have been added to a control system by running very long homerun cables from the control room to the remote application. If the remote application is, for example, a half of a mile away, the costs involved in running such a long cable can be very expensive. If multiple homerun cables have to be run to the remote application, the costs become even more prohibitive. Therefore, wireless communication is desirable. However, to minimize costs, it is also desirable to maintain existing control systems and communication protocols, to reduce the costs associated with changing existing systems to accommodate the wireless communication.